1. Field of the Invention
The present invention relates to an ignition system used for an igniter of an internal combustion engines, and particularly relates to an ignition system using a spark plug that can be effectively spark cleaned to have high resistance to fouling.
2. Description of the Related Art
In a parallel electrode type spark plug in which the distal end face of a central electrode opposes to a ground electrode, the voltage required is lower at the negative polarity than at the positive polarity and, hence, this type of spark plugs have been commonly used with an ignition system of negative polarity that applies a negative, high voltage to the central electrode. Accordingly, even in a semi-creep discharge type or an intermittent semi-creep discharge type of spark plugs that have a plurality of side ground electrodes provided to face the peripheral side of the central electrode, they are often used with the ignition system of negative polarity.
On the other hand, the internal combustion engine having high power and high performance, particularly the internal combustion engine for a motor cycle, has problems of breakage of the ground electrode due to mechanical vibration and excess heat of the ground electrode. It is difficult to apply the parallel electrode type spark plug to such an internal combustion engine. Further, the spark plug generally used has the main metallic shell having a small screw diameter such as M10S or M8S (JIS B8031) that is smaller than M14S. In this case, due to the size restriction of the main metallic shell, the cross sectional area of the ground electrode should be small. Consequently, the large amount of projection of the ground electrode like the parallel electrode type spark plug is difficult. Therefore, in such an internal combustion engine, a multi-electrode type spark plug in which a plurality of ground electrodes are provided so that they oppose to the peripheral side of the central electrode. In the multi-electrode type spark plug, if the ignition system in the negative polarity, it becomes a serious problem that the wearing of the central electrode which is collided with positive ions having heavy mass.
Further, since a problem of a carbon fouling during low load may easily occur, a semi-creep discharge type or an intermittent semi-creep discharge type of spark plugs have been used, in which a plurality of ground electrodes are provided to oppose to the peripheral side of the central electrode, and spark runs on the surface of the porcelain insulator between the central electrode and the ground electrode. In such a creepage spark discharge type spark plug, so-called xe2x80x9ccreepage spark dischargexe2x80x9d occurs, in which spark runs on the surface of the porcelain insulator. Accordingly, the channeling problem on the surface of the porcelain insulator easily occur. If the channeling proceeds, the heat resistance and the reliability of the spark plug are lowered to thereby shorten the life of the spark plug.
Generally, if the diameter of the central electrode is made small, the discharge voltage is lowered to thereby improve the ignitability. However, particularly, if the diameter of the central electrode of the spark plug is small which is used for the negative ignition system, the electrode is rapidly worn and the ground electrode is partially worn. Accordingly, the life of the spark plug is shortened and it can not be used for practical use.
The present inventors found that if a specific spark plug is used, the channeling resistance of the ignition system in positive polarity in which positive voltage is applied to the central electrode is superior in comparison with the general ignition system in negative polarity.
It is an object of the present invention to provide an ignition system used for high-power and high-performance internal combustion engines, capable of preventing the breakage and the excessive heating of the spark plug used therein, the system having excellent ignitability, high durability and long life. Further, it is another object of the present invention to provide an ignition system in which the fouling resistivity and ignitability of the spark plug used therein, and it is strong against the channeling to thereby elongate its life.
According to the first aspect of the present invention, an ignition system comprises: a spark plug comprising a central electrode, an porcelain insulator for holding the central electrode, a main metallic shell for holding the porcelain insulator, and a ground electrode electrically connected to the main metallic shell; and a positive voltage applying unit for applying a positive voltage to the central electrode of the spark plug in comparison with the ground electrode, so that an ignition high voltage is applied between the central electrode and the ground electrode; wherein a spark discharge gap is formed between the ground electrode and a distal end portion of the central electrode; and the ground electrode is positioned so that a rear edge of a mating face opposed to a peripheral side of the central electrode is positioned in the side of the distal end of the central electrode in comparison with an end face of the porcelain insulator.
In this ignition system, wherein the positive voltage applying unit may comprise an ignition coil having a primary coil and a secondary coil, and a primary current flowing in the primary coil of the ignition coil is stopped at a predetermined ignition period so as to apply the ignition high voltage is applied between the central electrode and the ground electrode of the spark plug. Further, in this ignition system, the positive voltage applying unit may further comprise a electric power source connected to the primary coil of the ignition coil and a control unit for controlling the ignition period of the primary current.
With this design, more of the spark jumps that are created by application of high voltage occur anywhere but along the surface of the porcelain insulator. In particular, according to an experiment on spark plugs that were operated with positive high voltage being applied to the central electrode, an appreciably increased proportion of the spark jumps occurred at the tip of the central electrode which was distant from the surface of the porcelain insulator. This enabled the spark plugs to be fired efficiently. What is more, the proportion of spark jumps along the surface of the porcelain insulator decreased dramatically during normal service and the chance of the spark of damaging the surface of the porcelain insulator is correspondingly reduced to make the spark plugs more resistant to channeling.
FIGS. 24 and 25 are section views of the distal end portion of a spark plug, showing how individual parts of the spark plug are electrified. FIG. 24 refers to the case of the present invention in which positive high voltage is applied to central electrode 2, and FIG. 25 refers to the conventional case where negative high voltage is applied to central electrode 2.
First, in the case of the present invention where central electrode 2 has positive polarity, the surface of porcelain insulator 1, particularly its end face, becomes negatively charged by dielectric polarization (see FIG. 24). As a result, the electric field near the front edge 11A of ground electrode 11 becomes stronger than the electric field near the rear edge 11B and spark is produced from the front edge 11A more frequently than from the rear edge 11B. However, the spark frequently produced at the front edge 11A is so much distant from the porcelain insulator 1 that channeling and other unwanted phenomena are less likely to occur.
Secondly, even if spark is produced at the rear edge 11B of the ground electrode 11, the negatively charged particles (e.g., electrons) that make up the greater part of the spark are repelled by the negative charge on the surface of the porcelain insulator 1 and the spark is more to prone to flash at a distance from the porcelain insulator 1 due to electrostatic repulsion. This lowers the probability of the occurrence of spark propagation along the surface of the porcelain insulator 1 and channeling due to spark attack is less likely to occur. On the other hand, in the conventional case operating at negative polarity, the surface of the porcelain insulator 1 becomes positively charged (see FIG. 25). As a result, spark is more prone to be attracted toward the surface of the porcelain insulator 1, increasing the chance of the occurrence of channeling.
The third factor to be considered is the difference in the mode of corona discharge that is a precursor to spark discharge. Spark discharge is usually preceded by corona discharge. It is generally held that the mode of corona discharge governs the behavior of the subsequently occurring spark discharge. Corona discharge behaves differently at positive and negative terminals. Take, for example, the case where a needle electrode opposed to a planer electrode is supplied with an increasing positive voltage. At low-voltage stage, only glow discharge occurs. As the applied voltage increases, the glowing tree stretches from the pointed tip of the needle electrode and moves briskly with hissing sound to make a shift to brush discharge. The first stage of brush discharge is brush corona, which develops into streamer corona more like spark discharge. If the needle electrode is supplied with negative voltage, the mode of discharge does not change as sharply as described above; with increasing voltage, a mode of discharge like glow corona is sustained near the pointed tip of the needle electrode and a glowing tree is not likely to appear.
This theory may be applied to describe the discharge that occurs between electrodes in a spark plug. First consider the conventional art case shown in FIG. 25 with negative voltage applied to the central electrode 2. The edges 11A and 118 of the ground electrode 11 may each be regarded as a positive pointed tip corresponding to the needle electrode. Brush discharge first occurs and the corona stretching from these edges reaches the central electrode 2 to cause xe2x80x9cbreakdownxe2x80x9d in spark discharge. Since the highest field intensity occurs near the rear edge 11B, the discharge path completed by the corona extending from that edge 11B is most likely to run along the surface of the porcelain insulator 1.
If the central electrode 2 is supplied with positive voltage as in the case of the present invention shown in FIG. 24, the front edge 2A of the central electrode 2 may be regarded as a positive pointed tip that corresponds to the needle electrode and the corona stretching from that edge reaches the ground electrode 11 to cause xe2x80x9cbreakdownxe2x80x9d. Since the ground electrode 11 is separated from the porcelain insulator 1 by the air, the concentration of the applied field is less subject to the influences of the surface charges on the porcelain insulator 1. Therefore, the discharge path completed by the corona somewhat xe2x80x9cfloatsxe2x80x9d, above the porcelain 1 to reduce the likelihood of channeling due to spark attack.
The fourth difference between FIGS. 24 and 25 is that the porcelain insulator 1 is damaged by different degrees depending on the direction of corona stretching. In the conventional art case shown in FIG. 25, corona stretches from the ground electrode 11 and the porcelain insulator 1 is directly subjected to the stress of intense field, increasing the chance of perforation (cavitation) of the porcelain insulator 1 by ion collision. On the other hand, in the case of the invention shown in FIG. 24, corona stretches from the central electrode 2 in contact with the porcelain insulator 1 and, in addition, the field on the porcelain insulator I is attenuated; this would reduce the likelihood of perforation of the porcelain insulator, thereby making it more resistant to channeling.
According to the second aspect of the present invention, the distal end portion of the central electrode, where the peripheral side of the central electrode is opposed to the ground electrode, preferably has a diameter of 2.0 mm or less.
With this design, in case of generating the creepage spark discharge, the spark clean efficiency for cleaning carbon fouling is improved, thereby improving the anti-fouling property. Further, if the endurance test is performed by using an ignition system in which the positive high voltage is applied to the central electrode of the multi-electrode spark plug, the electrode wear is remarkably decreased in comparison with the ignition system with negative polarity. The reason may be considered as follows. During discharge, positive ion existing between the discharge gap moves to the negative electrode and collides with it, and negative ion or electron moves to the positive electrode and collides with it. Positive ion is extremely heavier than negative ion or electron. Accordingly, the amount of the wear generated by the collision at the negative electrode with which positive ion collides is much larger than that at the positive electrode, as well as the temperature of the negative electrode is apt to be increased. If the central electrode is used with the negative polarity, the durability of the electrode, the diameter of which is less than 2 mm, is suddenly decreased. On the other hand, if the central electrode is used with the positive polarity, there is no such a trend. Consequently, the central electrode with the positive polarity can obtain the durability which is equal to or more than that of the central electrode having the diameter of 2 mm or more in the negative polarity. If the diameter of the central electrode is 1.9 mm or less, the effect of the positive polarity largely appears. Incidentally, if the central electrode is made too thin, it is excessively heated because of break of the balance between receiving heat and discharging heat. Accordingly, the diameter of the central electrode is desirably 0.4 mm or more, and more preferably, in the range of 0.6 mm to 1.8 mm.
Then, because the multi-electrode spark plug in which the required voltage necessary for spark discharge does not change much, it is possible to maintain low discharge voltage corresponding to the thin central electrode. Since the central electrode is thin, the ignitability is improved and the ground electrode is worn uniformly not unevenly. Further, the ground electrode is opposed to the peripheral side face of the central electrode, the projection amount of the ground electrode from the main metallic shell can be small, thereby preventing the breakage and excessive heating of the ground electrode.
According to the third aspect of the present invention, the diameter of the distal end portion of the central electrode is preferably in the range of 0.6 mm to 1.8 mm.
According to the fourth aspect of the present invention, the shortest distance (G) from the mating face of the ground electrode to the central electrode is preferably at least 1.5 times as long as the shortest distance (L) from the ground electrode to the porcelain insulator (1.5Lxe2x89xa6G).
If the end face of the porcelain insulator is fouled by carbon in this design (1.5Lxe2x89xa6G), the probability of creep discharge by a spark jump from the ground electrode to the end face of the porcelain insulator is increased and so is the probability that the carbon-fouled end face of the porcelain insulator is effectively spark cleaned. Hence, the spark plug is rendered highly resistant to fouling.
According to the fifth aspect of the present invention, the distal end face of the central electrode is preferably located between the front and rear edges of the mating face of the ground electrode.
With this design, the probability that spark jumps at the tip of the central electrode upon application of high voltage is so much increased that the firing efficiency of the spark plug is further improved. Further, the ground electrode is uniformly worn but is not worn unevenly much.
According to the sixth aspect of the present invention, the shortest distance (L) from the ground electrode to the porcelain insulator is preferably between 0.3 mm and 0.6 mm (0.3xe2x89xa6Lxe2x89xa60.6), and the shortest distance (G) from the mating surface of the ground electrode to the peripheral side of the central electrode is Gxe2x89xa6(2/3)L+1.0 (in millimeters).
Since 0.3xe2x89xa6L, the chance of the occurrence of so-called carbon bridge (a carbon lump or the like deposits between the porcelain insulator and the ground electrode to cause a short-circuit problem) is eliminated. on the other hand, Lxe2x89xa60.6, so the susceptibility to spark cleaning by creep discharge is by no means impaired. Further, it was verified by experiment that by satisfying the condition of Gxe2x89xa6(2/3)L+1.0 (in millimeters), the occurrence of spark jumps along the end face of the porcelain insulator can be reduced to attenuate channeling.
The reason for the attenuation of channeling may be explained as follows. If a sparkplug is installed on the actual engine and operated in a racing mode (engines runs at full speed under no load), the pressure in cylinders may sometimes become as high as 5 atmospheres when the spark plug fires a spark. The effect of pressure on discharge voltage is smaller in creep discharge along the surface of the porcelain insulator than in aerial discharge; therefore, under the contemplated high-pressure condition, spark jumps are likely to occur along the end face of the porcelain insulator even if its surface is not fouled by carbon. It should particularly be noted that under the contemplated high-pressure condition, more of the spark discharge that occurs is capacity-related to thereby provide high spark energy density. The spark of high energy density produces a greater amount of channeling and, hence, deeper channeling than the spark created under a low-pressure condition. Therefore, the occurrence of spark jumps along the surface of the porcelain insulator which in no way contribute to clean carbon fouling by spark is not preferred from an anti-channeling viewpoint. It has been found by experiment that if the shortest distance (L) from the mating surface of the ground electrode to the porcelain insulator and the shortest distance (G) from the mating surface of the ground electrode to the peripheral side of the central electrode are set to satisfy the relationship Gxe2x89xa6(2/3)L+1.0 (in millimeters), the occurrence of spark jumps along the surface of the porcelain insulator at high pressure can be effectively reduced.
According to the seventh aspect of the present invention, the central electrode preferably has a spark wear resistant member in at least a part of its distal end portion.
The spark wear resistant member may be made of any noble metal materials having higher melting points than Inconel which is a highly corrosion-resistant nickel alloy commonly used as an electrode material. Stated specifically, the spark wear resistant member may be made of noble metals, noble metal alloys, sintered noble metals and so forth that are exemplified by platinum (Pt), platinum-iridium (Ptxe2x80x94Ir), platinum-nickel (Ptxe2x80x94Ni), platinum-iridium-nickel (Ptxe2x80x94Irxe2x80x94Ni), platinum-rhodium (Ptxe2x80x94Rh), iridium-rhodium (Irxe2x80x94Rh), iridium-yttria (Irxe2x80x94Y2O3), etc.
With this design, the distal end portion of the central electrode where most of the spark jumps occur wears less and the life of the spark plug is accordingly prolonged. In addition, the tip of the central electrode is prevented from wearing to such an extent that it becomes less angular to have round edges and the concentration of spark jumps at the tip of the central electrode is accordingly maintained.
According to the eighth aspect of the present invention, the spark wear resistant member on the central electrode preferably extends to a position more rearward of the rear edge of the mating face of the ground electrode.
With this design, even if the spark jump from the ground electrode is flown to a position rearward of the spark plug by strong air flow in a combustion chamber, the spark jump achieved to the central electrode arrives at a portion where the spark wear resistant member exists to thereby prevent to wear the central electrode.
According to the ninth aspect of the present invention, the ground electrode preferably has a spark wear resistant member in at least a part of its mating face.
With this design, the mating face of the ground electrode positioned opposite the side of the central electrode to serve as a jump sparking face will wear less and the life of the spark plug is accordingly prolonged.